Implement unboxed sum primitive type

Summary:
This patch implements primitive unboxed sum types, as described in
https://ghc.haskell.org/trac/ghc/wiki/UnpackedSumTypes.

Main changes are:

- Add new syntax for unboxed sums types, terms and patterns. Hidden
  behind `-XUnboxedSums`.

- Add unlifted unboxed sum type constructors and data constructors,
  extend type and pattern checkers and desugarer.

- Add new RuntimeRep for unboxed sums.

- Extend unarise pass to translate unboxed sums to unboxed tuples right
  before code generation.

- Add `StgRubbishArg` to `StgArg`, and a new type `CmmArg` for better
  code generation when sum values are involved.

- Add user manual section for unboxed sums.

Some other changes:

- Generalize `UbxTupleRep` to `MultiRep` and `UbxTupAlt` to
  `MultiValAlt` to be able to use those with both sums and tuples.

- Don't use `tyConPrimRep` in `isVoidTy`: `tyConPrimRep` is really
  wrong, given an `Any` `TyCon`, there's no way to tell what its kind
  is, but `kindPrimRep` and in turn `tyConPrimRep` returns `PtrRep`.

- Fix some bugs on the way: #12375.

Not included in this patch:

- Update Haddock for new the new unboxed sum syntax.

- `TemplateHaskell` support is left as future work.

For reviewers:

- Front-end code is mostly trivial and adapted from unboxed tuple code
  for type checking, pattern checking, renaming, desugaring etc.

- Main translation routines are in `RepType` and `UnariseStg`.
  Documentation in `UnariseStg` should be enough for understanding
  what's going on.

Credits:

- Johan Tibell wrote the initial front-end and interface file
  extensions.

- Simon Peyton Jones reviewed this patch many times, wrote some code,
  and helped with debugging.

Reviewers: bgamari, alanz, goldfire, RyanGlScott, simonpj, austin,
           simonmar, hvr, erikd

Reviewed By: simonpj

Subscribers: Iceland_jack, ggreif, ezyang, RyanGlScott, goldfire,
             thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D2259

1 files changed, 4 insertions, 4 deletions

diff --git a/compiler/codeGen/StgCmm.hs b/compiler/codeGen/StgCmm.hs
index 73b9bf62ff..d6e0cf2f72 100644
--- a/compiler/codeGen/StgCmm.hs
+++ b/compiler/codeGen/StgCmm.hs
@@ -33,7 +33,7 @@ import HscTypes
 import CostCentre
 import Id
 import IdInfo
-import Type
+import RepType
 import DataCon
 import Name
 import TyCon
@@ -241,13 +241,13 @@ cgDataCon data_con
                 do { _ <- ticky_code
                    ; ldvEnter (CmmReg nodeReg)
                    ; tickyReturnOldCon (length arg_things)
-                   ; void $ emitReturn [cmmOffsetB dflags (CmmReg nodeReg)
-                                            (tagForCon dflags data_con)]
+                   ; void $ emitReturn [CmmExprArg (cmmOffsetB dflags (CmmReg nodeReg) (tagForCon dflags data_con))]
                    }
                         -- The case continuation code expects a tagged pointer
 
             arg_reps :: [(PrimRep, UnaryType)]
-            arg_reps = [(typePrimRep rep_ty, rep_ty) | ty <- dataConRepArgTys data_con, rep_ty <- flattenRepType (repType ty)]
+            arg_reps = [(typePrimRep rep_ty, rep_ty) | ty <- dataConRepArgTys data_con
+                                                     , rep_ty <- repTypeArgs ty]
 
             -- Dynamic closure code for non-nullary constructors only
         ; when (not (isNullaryRepDataCon data_con))